1. Field of the Invention
The present invention relates to battery separators and in particular, to filled fibrous battery separators for acid battery systems and methods of constructing the same.
2. Description of the Prior Art
In the construction of electrochemical storage batteries, separators are typically positioned between the positive and the negative plates in the cells of the battery to extend battery life and increase efficiency. A separator is a physical member or structure which retains the battery's electrolyte solution and provides a means for maintaining the anode plate in working chemical engagement with the cathode plate while maintaining physical separation between the plates. The separator is generally a porous membrane placed between the plates which freely permits electrolytic conduction in the cell while preventing contact between plates, which can result in shorting. It is generally desirable to employ separators with a relatively small pore size to retard the rate of active material from the battery plates penetrating into and through the separator, possibly resulting in shorting. Physical contact of the plates, which may be due to imperfections in the plate structure or due to warping or wrinkling of the plates during use, can be prevented by the interposition of a separator coextensive with the plates.
Battery separators which are commonly used today in lead acid batteries are commonly composed of phenol-aldehyde impregnated cellulose sheets, sintered polyvinyl chloride or highly-filled polyethylene. These battery separators are typically formed by preparing a mixture of fibers, a filler material and a binder and then molding or shaping the mixture into a sheet product, using techniques such as extrusion, injection molding, compression molding or processes resembling conventional papermaking. The shaping is typically performed utilizing elevated temperatures, pressure or shear force or a combination thereof.
If desired, ribs or other means may be used to add rigidity to the separator and to insure proper spacing between the plates. In addition, an electrolyte reservoir area may also be provided next to the positive plate for better conduction. Suitable ribs can be connected directly to the separator or the separator can be crimped or embossed during production to increase the spacing provided and create the reservoir.
Battery separators manufactured by W. R. Grace & Co. under the designations Darak 2000 and 5000 have utilized nonwoven webs which were filled with thermosetting resins (such as phenolics) and silica to make rigid leaf separators. The manufacturing process required that the resin be advanced in nondrying condition and then dried in order to retain the porous structure of the resin. The separator also included extruded ribs made from the same phenolic-silica mixture.
A separator manufactured by Standard Electric under the designation "Permalife" used latex and diatomaceous earth to fill and coat a special thick fiberglass sheet for use as heavy leaf separators. Other separators, such as those manufactured by U.S. Rubber and Amerace, were manufactured using a polymer-silica mixture, such as PVC-silica or hard rubber-silica.
There are many disadvantages to these prior separators. First, the separators are relatively thick with high electrical resistance, which limits their range of applications. This is especially true with the polymer-silica separators which often use high levels of polymer to increase mechanical strength, usually resulting in excessive electrical resistances. If relatively high levels of polymer are not used, the separators may have low mechanical strength, making them unfit for many applications. Second, phenolic binders have a tendency to make the separators brittle and incapable of being folded or enveloped around a plate for some applications. Third, phenolic binders often require special drying and curing conditions resulting in higher manufacturing costs.
It is an object of the present invention to provide a battery separator which overcomes the disadvantages of the prior battery separators.
It is also an object of the present invention to provide a battery separator which can be easily constructed out of readily purchasable fiber sheets rather than requiring specially-developed processes and machinery.
It is also an object of the present invention to provide a method of manufacturing battery separators which do not require specialized manufacturing conditions.
It is a further object of the present invention to provide a battery separator to which ribs can be easily attached, or can be crimped or embossed into a desired configuration.